The use of multi-axle tractor-trailers and other multi-axle vehicles in this and other countries has increased markedly over the past years. As trailers have been designed to carry heavier loads, axles have been spread wider apart on the trailers to conform with Federal and State bridge laws and the loadings permitted thereunder. The suspension systems for multi-axle trailers quite frequently include pneumatic, or "air bag" elements which cushion the shocks when pressured and permit rapid pressure release to remove the load from one of the axles when turning. If the pneumatic pressure is not exhausted from one or more of the several (only one of a two-axle trailer) axles, maneuverability of the trailer is lessened when approaching a loading dock, and torque is exerted on the trailer frame by the resistance to turning of the two axles, each of which have four tires in contact with the road surface. The greater the distance between the two axles, the greater the torque becomes. Sometimes spacing between axles is greater that ten feet.
The problem of torque in tight turns was overcome by raising all but one axle, thereby taking the load off all but one axle. In the case of air bag suspensions, the load is released by exhausting the air from the bags on all but one axle. In that way, maneuverability of the trailer is improved when the trailer is maneuvering at low speeds within a loading dock area or the like or during tight turns.
However, another problem has remained unsolved in such load-shifting devices. Operators may forget that they have shifted the load to only one axle, drive out of the truck yard and attain highway speed without re-inflating the air bags on the remaining axle or axles of the trailer. The overloading of a single axle in this manner can result in damage to the trailer frame and if extreme dynamic loading is experienced at highway speed. Tire wear is increased and braking capacity is decreased with an overloaded axle. Additionally, Federal and State laws govern the axle loads which may safely traverse bridges and certain highways. With all the trailer load on a single axle, these limits may well be exceeded. Therefore, it has become an important matter to make sure that loads are properly disributed on the multi-axle vehicles after loading and maneuvering operations are completed.